The possibility of manipulating magnetic systems without applied magnetic fields have attracted growing attention over the past fifteen years. The low-power manipulation of the magnetization, preferably at ultrashort timescales, has become a fundamental challenge with implications for future magnetic information memory and storage technologies. Here we explore the optical manipulation of the magnetization in engineered magnetic materials. We demonstrate that all-optical helicity-dependent switching (AO-HDS) can be observed not only in selected rare earth-transition metal (RE-TM) alloy films but also in a much broader variety of materials, including RE-TM alloys, multilayers and heterostructures. We further show that RE-free Co-Ir-based synthetic ferrimagnetic heterostructures designed to mimic the magnetic properties of RE-TM alloys also exhibit AO-HDS. These results challenge present theories of AO-HDS and provide a pathway to engineering materials for future applications based on all-optical control of magnetic order.

The Elliott-Yafet ͑EY͒ mechanism is arguably the most promising candidate to explain the light-induced ultrafast demagnetization dynamics in ferromagnetic transition metals on time scales on the order of 100 fs. So far, only electron-phonon ͑or impurity͒ scattering has been analyzed as the scattering process needed to account for the demagnetization. We show that an EY-like mechanism based on electron-electron scattering has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic Co and Ni films, without reference to a "phononic spin bath." Current research in femtosecond magnetism is concerned with elucidating the fundamental mechanisms of lightinduced spin dynamics as well as searching for potential applications in data processing. 1-3 Despite important experimental studies employing various time-resolved techniques, no consensus on a microscopic understanding of ultrafast magnetization dynamics in ferromagnets has emerged. Rather, demagnetization dynamics is typically described in the framework of the phenomenological three-temperature model. In this model, temperatures are assigned to the electron, lattice, and spin "subsystems," and the exchange of energy ͑and spin͒ is driven by the temperature differences between the respective subsystems. Although the threetemperature model provides an intuitive picture of demagnetization, its relation to the microscopic dynamics behind the demagnetization is still an active field of research.The most popular candidate 4 for the microscopic process behind light-induced ultrafast demagnetization is a mechanism of the Elliott-Yafet ͑EY͒ type. 5 In the EY mechanism, the demagnetization arises because, in the presence of the spin-orbit ͑SO͒ interaction, spin is not a good quantum number, so that any momentum-dependent scattering mechanism changes the spin admixture when an electron is scattered from state ͉k ជ ͘ to ͉k ជ + q ជ͘. So far, the scattering processes responsible for the EY mechanism have been assumed to be ͑quasi͒elastic electron-phonon and electron-defect scattering in several theoretical and experimental studies. 4,6-9 Unlike these papers, we analyze the ultrafast demagnetization in ferromagnetic metals due to an EY-like mechanism based exclusively on electron-electron Coulomb scattering. This scattering mechanism is not ͑quasi͒elastic, so that the available phase space for transitions from minority to majority bands is much larger than for electron-phonon scattering, which can only cause transitions near points in the Brillouin zone where the bands are energetically close. As a proof of principle for the importance of electron-electron scattering for the demagnetization, we demonstrate quantitative agreement for the demagnetization time and magnetization quenching between time-resolved magneto-optical Kerr effect ͑TR-MOKE͒ measurements on Co and Ni, and numerical results based on the EY mechanism due to electron-electron scattering.To resolve the electronic demagnetization dynamics on ultrafast time scales, we calculate the...

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The microscopic mechanisms responsible for the ultrafast loss of magnetic order triggered in ferromagnetic metals by optical excitation are still under debate. One of the ongoing controversies is about the thermal origin of ultrafast demagnetization. Although different theoretical investigations support a main driving mechanism of thermal origin, alternative descriptions in terms of coherent interaction between the laser and the spin system or superdiffusive spin transport have been proposed. Another important matter of debate originates from the experimental observation of two time scales in the demagnetization dynamics of the 4f ferromagnet gadolinium. Here, it is still unclear whether it is necessary to invoke two distinct microscopic mechanisms to explain such behavior, or if one single mechanism is indeed sufficient. To uncover the physics behind these two unsolved issues, we explore the dependence of ultrafast-demagnetization dynamics in nickel through a survey of different laser intensities and ambient temperatures. Measurements in a large range of these external parameters are performed by means of the time-resolved magneto-optical Kerr effect and display a pronounced change in the maximum loss of magnetization and in the temporal profile of the demagnetization traces. The most striking observation is that the same material system (nickel) can show a transition from a one-step (one time scale) to a two-step (two time scales) demagnetization, occurring on increasing the ambient temperature. We find that the fluence and the temperature dependence of ultrafast demagnetization-including the transition from one-step to two-step dynamics-are reproduced theoretically assuming only a single scattering mechanism coupling the spin system to the temperature of the electronic system. This finding means that the origin of ultrafast demagnetization is thermal and that only a single microscopic channel is sufficient to describe magnetization dynamics in the 3d ferromagnets on all time scales.

Magnetization reversal using circularly polarized light provides a new way to control magnetization without any external magnetic field and has the potential to revolutionize magnetic data storage. However, in order to reach ultra-high density data storage, high anisotropy media providing thermal stability are needed. Here, we evidence all-optical magnetization switching for different TbxCo1-x ferrimagnetic alloy composition and demonstrate all-optical switching for films with anisotropy fields reaching 6 T corresponding to anisotropy constants of 3x106 ergs/cm3. Optical magnetization switching is observed only for alloys which compensation temperature can be reached through sample heating